Cosmic neutrino background

Lore & Background

The early universe consisted of a very hot dense plasma that expanded and cooled. Particles collided and reacted, maintaining equilibrium. Around 1 second after the Big Bang, the equilibrium among electrons, positrons, and neutrinos was disrupted. Annihilation reactions such as e⁻ + e⁺ ⟷ νₑ + ν̄ₑ stopped because expansion left neutrinos too far apart to find each other, a state known as decoupling. The neutrinos then free-streamed through the universe.

Reader's Guide

The cosmic neutrino background is significant because it provides a direct window into the universe when it was only one second old, far earlier than the cosmic microwave background. Although neutrinos rarely interact with matter, making direct detection extremely difficult—their energies are around 10¹⁰ times smaller than even high-energy neutrinos—Big Bang cosmology makes many predictions about the CνB. There is very strong indirect evidence that it exists.

Did You Know?

• The cosmic neutrino background decoupled when the universe was just one second old.
• Its current temperature is estimated at roughly 1.95 K.
• Neutrinos in the CνB have energies around 10⁻⁴ to 10⁻⁶ eV.
• The CνB may not be directly observed in detail for many years, if at all.